Ch. Ill, 11] COLORATION OF LEAVES 89 



much as the photographer's ruby hght cuts off the same 

 rays which would spoil his plate in development ; and thus 

 is tided over the time prior to the full formation of the 

 chlorophyll, which incidentally acts as a sufficient protec- 

 tion. It has also been supposed that the absorbed light 

 is converted into heat, and used to warm the young parts 

 and thus promote their development. The latter explana- 

 tion would account for the prevalent red color in the mosses 

 of open bogs, which are notoriously cold places, ^'arious 

 explanations have also been ofi'ered for the deep red of 

 the under sides of leaves in some tropical plants, and 

 for the brilliant hues of the toadstools. But the e^-idence 

 in these cases does not stand our earlier-cited test for sci- 

 entific truth (page 13), which shows how much we have 

 still to learn about some of the commonest phenomena. 

 The case is ciuite different, however, vrith the colors in flowers 

 and fruits, for here the evidence demonstrates functional use, 

 as ■niU later appear. A functional use seems also reasonably 

 clear in the beautiful rose-red Alga called "sea mosses," 

 where the red screen (here, however, not erj-throphyll. but 

 another red pigment) probably aids the underlying chloro- 

 phjdl in a better utilization of the sunlight as altered by its 

 passage through the sea water. 



Second in prominence of the non-green colors of living 

 leaves is ijellou:. Indeed, the normal green color of leaves is 

 not a perfectly pure green, but tends a trifle towards yellow, 

 which, however, is only rarely pronoimced in healthy leaves. 

 It occurs occasionally in small blotches and stripes in wild 

 plants, from which it has been much developed under cul- 

 tivation in some variegated leaves, notably in yellow vari- 

 eties of Coleus. It is more commonly associated with 

 waning Aitahty of the leaf, whether through old age, or 

 insufficient light, or the action of parasites, or (and above 

 all) the fall of the leaves in autumn. It is due to the presence 

 along -with the chlorophyll, of a mixture of yellow pigments, 

 descriptively called xaxthophtll, and composed chiefly 



